Direct-view half-tone storage tube



May 1, 1962 K. R. HEssE ETAI. 3,032,673

DIRECT-VIEW HALF`-TONE STORAGE TUBE Filed Jan. 2, 1958 2 Sheets-Sheet 1 May l, 1962 K. R. Hl-:ssE ETAL 3,032,673

DIRECT-VIEW HALF-TONE sToRAGE TUBE Filed Jan. 2, 1956 2 Sheets-sheet 2 AI-I-I I l l l l l In A -I-.Iln l l l l l l l l I I Il l Il United States fatent hline 3,032,673 Pgatented May 1, 1952 3,032,673 DIRECT-VIEW HALF-TONE STORAGE TUBE Kenneth R. Hesse, Granada Hills, and Leon S. Yaggy,

Los Angeles, Calif., assignors to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Filed Jan. 2, 1958, Ser. No. 706,842 3 Claims. (Cl. 313-71) This invention relates to direct-viewing half-tone storage tubes and more particularly to a half-tone directviewing storage tube incorporating an electron gun having a substantially greater video drive requirement.

A conventional direct-viewing half-tone storage tube comprises, for example, a viewing screen, means for directing ilood electrons uniformly over the viewing screen, a storage screen disposed adjacent to and coextensive with the viewing screen in the path of the ood electrons, and means including an electron writing gun for producing a charge replica on the storage screen thereby to control the ow of ilood electrons to the viewing screen to produce a visual image of the charge replica. Until the present time, the electron writing gun employed in such half-tone storage tubes delivered many microamperes of beam current when the control grid was driven but a few volts from cutoff. At moderate scanning rates of the writing 4beam on the storage screen, such as are encountered in typical radar-type applications, the beam current required to cause the brightness of the display to go all the way from the black level to the white level was very small. Thus, in order to avoid displaying all information at full brightness instead of at the relative levels of brightness desired, it became necessary to operate conventional electronwriting guns with very small signals on their control grids. Such very small signals as required by these conventional electron writing guns are very difcult to control in that the slightest fluctuation in the signal amplitude or the direct-current level on the control grid causes loss of half-tones.

In accordance with the present invention, the abovementioned diiculty is avoided by employing an electron writing gun which includes means for interceptng a substantial portion of the current of the writing beam subsequent to its being modulated by the application of a normal video signal on its control grid. More particularly, an electrode with an array of small apertures is inserted in the path of the electron beam subsequent to its intensity being determined by the amplitude of the video signal whereby only a predetermined percent of the electrons in the beam are permitted to pass through and arrive at the storage screen. The percent of the beam passing through the electrode is determined by the effective transparency thereof which in general will be determined by the ratio of the area of the apertures to the total area of the electrode. The size of the apertures is small compared to the cross-sectional area of the writing beam and the number of apertures within the cross-sectional area is large enough so that there are no abrupt changes in the resulting beam current due to separate contributions of individual apertures when the beam current is varied with concomitant variations in cross-sectional area. It is thus apparent that the direct-viewing half-tone storage tube of the present invention can be operated withoutttaking special care or without employelectron writing gun adapted to operate with a substantially increased video signal drive.

Still another object of the present invention is to provide an electron writing gun for use in a storage tube, the electron gun having means to intercept a predetermined portion of the current constituting the electron writing beam subsequent to the varying of the intensity of the beam with a drive signal.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent by reference to the following description taken in conjunction with the accompanying drawings, wherein:

FIG. 1 illustrates a cross-sectional schematic view of Y the direct-viewing half-tone storage tube of the present invention;

FIG. 2 shows a cross-sectional perspective view of the electron Writing gun in the device of FIG. 1; and

FIGS. 3-5 illustrate plan views of alternate forms of an electrode in the electron gun of FIG. 2.

Referring now to FIG. l of the drawings, there is shown a cross-sectional schematic view of thedirect-viewing storage tube of the present invention which comprises an evacuated envelope 10. This evacuated envelope 10 is constituted of an enlarged cylindrical portion 11 with a at faced portion 12 at one extremity thereof, and an enclosing portion 13 at the opposite extremity of the cylindrical portion 11. The enclosing portion 13 of the evacuated envelope 10 includes an indented or well portion 14 in the center thereof opposite the at faced portion 12 and an arm portion 15 having a longitudinal axis which intersects with the longitudinal axis of the cylindrical portion 11 at an oblique angle, as shown in the drawing.

The indented or well portion 14 of the evacuated envelope 10 accommodates a ood gun 16 which includes a flood cathode 18. In operation, the flood cathode 18 of flood gun 16 is maintained at a substantially fixed reference potential such as, for example, ground by means of an appropriate connection thereto. The ood gun 16 in conjunction with electrodes 20, 22 directs a ow of flood electrons uniformly towards the at faced portion 12 of the kevacuated envelope 10. Electrodes 20, 22 are disposed successively about the inner periphery of cylindrical portion 11 of evacuated envelope 10 in proceeding, respectively, from the flood gun 16 to the flat faced portion 12. In addition, electrode 20 extends over the enclosing portion 13 and into the arm portion 15 to provide an equipotential region in the left portion of evacuated envelope 10, as viewed in the drawing. In operation, electrodes 20, 22 are maintained at potentials of the order of +l0 volts and +40 volts with respect to ground, respectively.

Over the entire inner surface of at faced portion 12 of evacuated envelope 10 there is disposed a viewing screen 24 which includes a fluorescent screen 25 covered with a thin metallic coating 26 of, for example, aluminum. Proceeding towards the ood gun 16 there is disposed adjacent to and coextensive with the viewing screen 24, in the order named, a storage screen 28 and a collector grid 30. The storage screen 28 is o-f the type which may comprise, for example, an electro-formed metallic screen 31 on which there is evaporated a thin film 32 of, for example, magnesium fluoride on the side thereof facing ilood gun 16 to provide a storage surface. In operation, the metallic coating 26 of viewing screen 24, the metallic screen 31 of storage screen 28 and the collector grid 30 are maintained at potentials of the order of +5000 volts, +5 volts and +120 volts with respect to ground, respectively. Further, the metallic screen 31 is connected to its respective source of potential through a resistor 33 and positive pulses generated thereacross by means of appropriate connections to a pulse generator 34. Under normal operation, these pulses will have positive excursions of from to 10 volts.

The arm portion l5 of evacuated envelope l0 accommodates an electron gun 35 for producing an electron writing beam in accordance with the present invention, and a deflection apparatus 36 for directing the electron writing beam towards selected elemental areas of storage screen 28. The electron gun 35 includes a cathode 37 and an intensity control grid 38 which are maintained, respectively, at quiescent potentials of the order of 3000 volts and -3025 volts with respect to ground, respectively. This may be accomplished, for example, by connecting the control grid 38 through a resistor 39 to the negative terminal of a variable potential source itil, the positive terminal of which is connected to both cathode 37 and the negative terminal of a 3000 volt battery 4l, the positive terminal of which is referenced to ground. Signals provided by a signal source 39 are applied to control grid 3S of electron gun 35 through a capacitor 42.

A more detailed description of the electron gun 35 is presented in conjunction with FIG. 2 of the drawings. Referring to FIG. 2, the cathode 37 may, for example, be provided by a sleeve-type cathode which is heated by means of ya heater filament 44. The control grid 38 comprises a metallic disc with a circular aperture disposed concentrically about a center line through the emitter surface of cathode 37. Along with the control grid 38, there are disposed, in the order named, about the path of the electron beam in the direction of electron flow, an anode 46 and a lens assembly 4S. The anode 46 constitutes an elongated cylindrical portion 49 which has an apertured appendage 50 on the extremity thereof adjacent the control grid 38. The lens assembly 48, on the other hand, comprises narrow cylindrical members 52, 53 disposed concentrically about the path of the electron beam which members 52, 53 have a diameter of the order of two times the diameter of cylindrical portion 49 of anode 46. In addition, circular discs 54, 55 are appended to the opposite extremities of cylindrical members 52, 53, respectively, with circular apertures therethrough to accommodate the ow of the electron beam. Circular disc 54 is, in turn, attached to the adjacent extremity of the cylindrical portion 49 of the anode 46. Lens assembly 48 also includes a cylindrical member 56 interposed between and of the same diameter as the cylindrical members 52, 53. In operation, the anode 46 and cylindrical member 52, the cylindrical member 56, and the cylindrical member 53 are maintained at potentials of the order of l0 volts, 2400 volts and l() volts, with respect to ground, respectively.

In addition to the above, an electrode S8 having an array of holes or apertures is disposed over the aperture in disc 54 in the path of the electron beam. The electrode 58 may, for example, constitute an electroformed mesh 58a (see FIG. 3), a photo-etched sheet 58b (see FIG. 4), a woven screen 58C (see FIG. 5) or any construction which results in a sheet which is opaque to electrons except at small, evenly distributed areas. In general, the transparency of electrode 58 is of the order of 1%. Further, the electrode S8 can be located at any plane normal to the path of the beam where the beam is in a defocused condition and subsequent to its intensity being determined by the amplitude of a video drive signal, namely, along the path of the electron beam subsequent to its passing through the control grid 38. Due to the low relative transparency, it is highly desirable that the electrode 58 is not placed in a position such that it intercepts a portion of the flow of flood electrons. As mentioned above, a convenient location for the electrode 58 is at the aperture in disc 54 through which the beam passes before entering the focusing lield. It is, of course, apparent that either elecag. trostatic or magnetic focusing may be used. In addition to the above, the electrode S8 may also be located at a plane after the beam emerges from the focusing eld before deflection takes place, or even after some deflection has occurred.

In the operation of the half-tone direct-viewing storage tube of the present invention, the inherent capacitance between the metallic screen 31 and the storage surface of storage screen 28 initially raises the potential existing on the storage surface to |5 volts with respect to ground. The ood electrons emanating from flood cathode i8 maintained at ground potential immediately commence discharging the storage surface to ground potential. Subsequent pulsing of the metallic screen 31 of storage screen 28 periodically raises the potential of the storage surface 5 to 10 volts positive with respect to ground, depending on the amplitude of the pulse excursions. During these periodic pulse intervals when the storage surface is positive with respect to ground the flood electrons impnge on the storage surface to charge it towards ground potential. Thus, the storage surface assumes a quiescent potential that is negative with respect to ground to the extent of the pulse amplitude. At this quiescent negative potential, substantially all the ood electrons are repelled and thus prevented from penetrating through the storage screen 28 to the viewing screen 24.

A charge replica is produced next on the storage surface by the application of the signal provided by the signal source 39 to the intensity control grid 38 of electron Writing gun 3S and scanning the electron beam over the storage surface in synchronism therewith by means of the deliection apparatus 36. In order to produce a presentation on the viewing screen 24 with half-tones, it is generally necessary that the variation in the potentials constituting the charge replica be restricted to from l to 3 volts. In prior art devices, this restriction made it necessary to bias the electron writing beam to substantially cut-olf whereupon only a comparatively low amplitude video drive signal was required to produce a charge replica having the desired variation in potentials. In accordance with the present invention, however, the intensity of a substantial ow of electrons may first be controlled by intensity grid 38 of electron gun 35 after which a substantial portion of the electron beam is intercepted by the perforated or foraminous electrode 58. Thus it is evident that a video drive signal of convenient amplitude may be employed consistent with producing a charge replica having the requisite potential variations of only 1 to 3 volts thereby to preserve the half-tones in the visual presentation. The continued pulsing of the metallic screen 31 by the pulse generator 34 will enable the flood electrons to proceed to erase the charge replica thus produced.

What is claimed is:

l. A half-tone direct-viewing storage tube comprising a viewing screen; means for directing flood electrons uniformly towards said viewing screen; a storage screen disposed adiacent to and coextensive with said viewing screen in the path of said flood electrons; and means for producing a charge replica on said storage screen thereby to control the flow of said flood electrons to said viewing screen, said last-named means including a source of electrons, means for directing said electrons along a predetermined path thereby to produce an electron beam, said electron beam being defocused over at least a portion of the length of said predetermined path, means disposed adjacent said source of electrons and prior to said portion of said predetermined path for controlling the intensity of said electron beam in response to an electrical signal, and a substantially uniformly multi-apertured electrode inserted in said electron beam along said portion of said path wherein said electron beam is defocused and at a point therealong not exposed to ood electrons thereby to decrease the intensity of said elec- 5 6 tron beam, and means for directing the entire portion References Cited in the le of this patent of the electron beam penetrating through said multi-aper- UNITED STATES PATENTS tured electrode towards successively selected circularshaped elemental areas of said storage screen. 2318423 Samuel May 4 1943 2. The half-tone direct-Viewing storage tube as dened 5 2728872 Smith Dec' 27 1955 in claim 1 wherein said multi-apertured electrode has a 2761988 MCNaney Sept' 4 1956 transparency of less than 5%. 2,790,929 Herman et al. Apr. 30, 1957 3. The half-tone direct-viewing storage tube as de- 2826716 McNaney Mar' 11 1958 fined in claim l wherein said multi-apertured electrode 2880342 Henkel Mar' 31 1959 constitutes a thin metallic plate With a plurality of aper- 10 FOREIGN PATENTS tures therein, the area ofindividual ones of said aper- 521,803 Great Britain May 31 1940 tures being substantlally less than the cross-sectlonal area 534,215 Great Britain Mar. 3 1941 of said defocused electron beam. 

